Thursday, October 28, 2010

Tsunami Region Ripe for Another Big Quake, Study Says

The earthquake that triggered the December 26 tsunami has increased stress on nearby faults, making another major South Asian quake more likely, scientists reported today.

The magnitude 9 earthquake was centered off the west coast of Sumatra, an Indonesian island. The quake shifted nearly 97,000 square miles (250,000 square kilometers) of terrain along the Sunda trench subduction zone, where the Indonesian and Australian tectonic plates dive beneath the Burma tectonic plate.

In places, the earthquake caused the ocean floor to shift as much as 65 feet (20 meters). This displacement triggered the tsunami in the Indian Ocean that, according to the United Nations, killed nearly 300,000 people in South Asia and East Africa.

A team of seismologists led by John McCloskey at the University of Ulster in Northern Ireland, say more shaking could be in store.

McCloskey and colleagues found that the so-called Sumatra-Andaman earthquake increased the stress on adjacent sections of the Sunda trench and the nearby Sumatra fault, which runs the length of Sumatra.

The team cannot predict when another earthquake will occur. But they say the increased stress, combined with historical evidence, raises the likelihood of another big quake in the region. History, they say, shows that one earthquake tends to follow another in the same region.

"People believe lightning never strikes twice in the same place," McCloskey said. "Earthquakes do. Earthquakes cluster in space and time. When you get an earthquake, you are more likely to get another, and our calculations show the stress interaction [in South Asia] is very high."

McCloskey and colleagues Suleyman Nalbant and Sandy Steacy published their calculations in tomorrow's issue of the science journal Nature.

The greatest risk, the scientists say, is for a magnitude 7.5 earthquake on the Sumatra fault near the already devastated town of Banda Aceh. The town is northeast of the epicenter of the December 26 earthquake.

Stress Interaction

McCloskey and colleagues used a map of the slip and displacement from the Sumatra-Andaman earthquake to calculate the stress buildup on the Sunda trench and neighboring Sumatra fault.

The map was created by Chen Ji, a seismologist at the California Institute of Technology in Pasadena. According to Ji's work, portions of the ocean floor along the Sunda trench moved as much as 65 feet (20 meters) in an area stretching from the epicenter north for about 310 miles (500 kilometers).